Opera has released an alpha build of their upcoming 9.5 release. "Following the release of Opera 9 last year, we re-wrote Opera's rendering engine for the coming Opera 9.5 release. As a result, Opera 9.5 contains more than a year's worth of speed, standards and performance improvements."

1) Opera will not respect your qtconfig or Kcontrol font settings but can change the font face and size under Tools -> Preferences -> Advanced -> Fonts. Also make sure you are using a "shared" QT version of Opera, not a static one (Slackware should be packaging the shared version).

Here is a screen shot of the new Opera 9.50a1 amd64 build on my Gentoo/Gnome desktop so you can see what can be achieved:

2) Flash will work on Opera provided Opera can find the flash plug-in library, which is probably somewhere in /opt. You really should file a distro bug for this since it's a packaging issue. You can also point Opera to the correct location in Tools -> Preferences -> Advanced tab -> Content -> Plugin Options